Acoustic transducer with gap-controlling geometry and method of manufacturing an acoustic transducer

ABSTRACT

A transducer of the preferred embodiment including a transducer and a plurality of adjacent, tapered cantilevered beams. Each of the beams define a beam base, a beam tip, and a beam body disposed between the beam base and the beam tip. The beams are arranged such that each of the beam tips extends toward a common area. Each beam is joined to the substrate along the beam base and is free from the substrate along the beam body. A preferred method of manufacturing a transducer can include: depositing alternating layers of piezoelectric and electrode onto the substrate in block, processing the deposited layers to define cantilever geometry in block, depositing metal traces in block, and releasing the cantilevered beams from the substrate in block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application number 61/470,384 entitled “Acoustic Sensor with Gap-Controlling Geometry and Method of Manufacturing an Acoustic Sensor” and filed 31 Mar. 2011, the entirety of which is incorporated herein by this reference.

BACKGROUND

The current trend in acoustic transducer technology has been toward smaller microphones. Currently, electret microphones based on thin, charge-carrying membranes have been used in most applications. However, these microphones suffer from degradation after exposure to high temperatures. Capacitive MEMS microphones are gaining popularity because they can withstand the high temperatures experienced during solder-reflow and, therefore, reduce manufacturing cost. Piezoelectric MEMS microphones have been researched for more than 30 years and can potentially combine the advantages of electret microphones and MEMS capacitive microphones in a cost-effective manner. Unfortunately, piezoelectric MEMS microphones traditionally suffer from high noise floors due, in part, to diaphragm tension caused by residual stress in thin films. For example, diaphragm microphones are constrained on all edges, which leads to high diaphragm tension that results in decreased sensitivity. Conventional cantilevered designs, such as rectangular cantilever beam microphones, also suffer from the effects of residual stress despite being substantially released from the surrounding substrate; instead, the small amount of residual stress causes the cantilever to bend away from the substrate plane, either upwards or downwards. This bending causes the gap around the cantilever to increase, decreasing the acoustic resistance and resulting in an undesirable decrease in low-frequency sensitivity.

Thus, there is a need in the piezoelectric MEMS acoustic transducer field to create a new and useful acoustic transducer with low frequency sensitivity despite residual stresses.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a representative prior art transducer.

FIG. 2 is a plan microscopic view of a cantilevered beam in accordance with a preferred embodiment of the present invention.

FIGS. 3A, 3B, 3C, 3D, and 3E are schematic representations of a first, second, third, fourth, and fifth preferred embodiments of the present invention, respectively.

FIG. 4 is a schematic perspective view of a portion of a transducer in accordance with the preferred embodiments of the present invention.

FIG. 5 is a flowchart depicting a method of manufacturing a transducer in accordance with a preferred embodiment of the present invention.

FIGS. 6A-6H are schematic representations of an example transducer manufactured in accordance with one variation of the method of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art of MEMS acoustic transducers to make and use this invention.

Acoustic Transducer

As shown in FIGS. 2, 3, and 4, a transducer 100 of the preferred embodiment can include a substrate 160 and multiple cantilevered beams 120 arranged in a gap-controlling arrangement, each beam 120 including gap-controlling geometry 130. The geometry and arrangement of the preferred transducer 100 allows the gap resistance to be controlled, which in turn enables control of the frequency below which the response of the microphone “rolls off” or diminishes. The transducer 100 is preferably a piezoelectric transducer, but may alternatively be a capacitive transducer, an optical transducer (e.g. optical acoustic sensor), or any other suitable transducer that suffers from stressed cantilevers. The preferred transducer 100 is preferably an acoustic transducer, more preferably an acoustic sensor (i.e. microphone), but may alternatively be driven with a voltage or current and used as a speaker. The preferred transducer 100 is preferably incorporated into consumer electronics, such as mobile phones, but may be used in medical applications (e.g. hearing aids), photoacoustic detection, ultrasonic applications, or any other transducer-based application as a sensor or speaker. The cantilever arrangement of the preferred transducer 100 functions to limit the gap size to the separation distance between adjacent cantilevered beams 120. This is in contrast with conventional designs, wherein the residual stress within the cantilevered beams cause the beams to deflect drastically after separation, sometimes as much as the distance between the substrate and the cantilevered beams. In the transducer 100 of the preferred embodiments, the proximity between the multiple cantilevered beams 120 on the substrate 160 causes the residual stresses between the beams 120 to be similar. The similar stress profiles between the adjacent cantilever beams result in similar cantilever curvatures, which, in turn, limits the gap size to the separation distance between adjacent cantilevered beams 120.

As shown in FIG. 4, the preferred transducer 100 can include a substrate 160. The substrate 160 of the preferred transducer 100 functions to support the transducer 100 during the manufacturing process, as well as to support the cantilevered beam 120 of the transducer 100 during operation. In one variation of the preferred transducer 100, the substrate 160 is composed at least in part of silicon or any suitable silicon-based compound or derivative (e.g. Si wafer, SOI, polySi on SiO₂/Si, etc.) Alternately, the substrate 160 can be composed at least in part of fiberglass, glass, or any suitable combination of materials. In another variation of the preferred transducer 100, the substrate 160 is substantially removed from the active area of the cantilevered beams 120 during the manufacturing process such that the cantilevered beams 120 have maximized travel and response.

As shown in FIGS. 2, 3, and 4, the preferred transducer 100 can include at least one cantilevered beam 120. More preferably, the preferred transducer 100 can include a plurality or array of cantilevered beams 120 arranged in a suitable geometry, such as those shown for example in FIGS. 3A to 3E. The cantilevered beams 120 of the preferred transducer 100 function to transform acoustic pressure into electronic signals. The cantilevered beams 120 preferably include gap-controlling geometry 130 that minimizes the resultant gaps between each of the cantilevered beams 120 with a larger plurality or array. The gap-controlling geometry 130 of the cantilevered beam 120 preferably includes a tip 132 and a base 134. In a preferred cantilevered beam 120, the tip 132 is substantially smaller than the base 134, such that the cantilevered beam 120 tapers from the base 134 to the tip 132. Except for the base 134, which is preferably substantially coupled to the substrate 160, the whole of the cantilevered beam 120 is preferably substantially detached from the surrounding substrate 160 such that it can expand or contract to relieve imparted stress.

As shown in FIGS. 3A-3E, the cantilevered beam 120 preferably has a substantially pointed gap-controlling geometry 130, such that the base 134 is substantially wider than the tip 132 along a direction perpendicular to a direction of travel of the cantilevered beam 120. For example, one variation of the preferred transducer 100, the cantilevered beam 120 shown in FIGS. 3A, 3B, and 3C can have a substantially triangular geometry. In another variation of the preferred transducer 100, the cantilevered beam 120 can have a circular sector- or wedge-like geometry 130 with a substantially curved base 134 as shown in FIG. 3D. In another variation of the preferred transducer 100, the cantilevered beam 120 can have a square geometry as shown in FIG. 3E. In other variations of the transducer 100 of the preferred embodiment, the cantilevered beam 120 can have any suitable geometry in which the tip 132 is narrower in breadth than the base 134 along a direction perpendicular to the direction of travel of the cantilevered beam 120. Suitable alternative geometries can include any type of polygonal shape or circular sector, and each cantilevered beam 120 with an array can have substantially identical or substantially non-identical geometries. The length of the cantilevered beam 120 is preferably adjusted to match the desired resonant frequency of the microphone, but may alternatively be longer or shorter. The base 134 is preferably twice as wide as the length, but may alternatively have any width that allows for the desired transducer 100 outer perimeter 102 geometry to be achieved.

The cantilevered beam 120 is preferably made from alternating piezoelectric and electrode layers 142. The piezoelectric layers 144 can function to convert applied pressures to voltages, and the electrode layers 142 can function to transmit the generated voltages to an amplifier such as a JFET, a charge amplifier, or an integrated circuit. The piezoelectric layers 144 preferably include aluminum nitride (AlN) due to its CMOS compatibility, but may alternatively include lead zirconate titanate (PZT), zinc oxide (ZnO), polyvinylidene fluoride (PVDF), lead mangnesium niobate-lead titanate (PMN-PT), or any other suitable piezoelectric material. The electrode layers 142 preferably include molybdenum (Mo), titanium (Ti), aluminum (Al), or platinum (Pt), but may alternately include any other suitable electrode material. The cantilevered beam 120 preferably includes two piezoelectric layers 144 interspersed between three electrode layers 142. However, the cantilevered beam 120 can include three piezoelectric layers 144 interspersed between the three electrode layers 142, include only three total layers (a first electrode layer 142, a first piezoelectric layer 144, and a top electrode layer 142), or any number of layers in any suitable permutation of electrode layers 142 and piezoelectric layers 144. Preferably, the cantilevered beam 120 incorporates at least one piezoelectric layer 144 and one electrode layer 142. In one example configuration, the electrode layers 142 preferably cover substantially two-thirds of a substantially triangular cantilevered beam 120 area to minimize the noise floor, but can alternately cover more or less of the cantilevered beam 120 depending on the cantilevered beam 120 geometry. Additionally, although each electrode layer 142 preferably defines only one independent electrode per electrode layer 142, the electrode layers 142 may be patterned to define multiple independent electrodes per electrode layer 142. The electrode layers 142 are preferably coupled together by metal traces in series, but may be coupled in parallel or both in series and in parallel.

As shown in FIG. 3, the preferred transducer 100 is configured such that substantially identical cantilevered beams 120 are arranged such that the tips 132 meet in a common area substantially near the center of the acoustic transducer 100. Preferably, each of the free edges 136 of each cantilevered beam 120 are preferably each parallel to a free edge 136 of the adjacent cantilevered beam 120. The respective bases 134 of the cantilevered beams 120 preferably form an outer perimeter 102 in the shape of a regular polygon and/or circle. The outer perimeter 102 of the transducer 100 is preferably a square wherein the transducer 100 preferably incorporates four cantilevered beams 120 (shown in FIG. 3A), but the outer perimeter 102 may alternately be a circle wherein the acoustic transducer incorporates any suitable number of wedge-like cantilevered beams 120 (shown in FIG. 3D), a triangle wherein the transducer 100 preferably incorporates three cantilevered beams 120 (shown in FIG. 3B), an octagon wherein the transducer 100 preferably incorporates eight cantilevered beams 120, a hexagon wherein the transducer 100 preferably incorporates six cantilevered beams 120 (shown in FIG. 3C), or any geometric shape incorporating any requisite number of cantilevered beams 120.

In the preferred transducer, the gaps between the cantilevered beams 120 are approximately less than 1 micron during manufacture, but may be slightly larger. After manufacture, the gaps between the cantilevered beams 120 are preferably maintained under 1 micron, but may be significantly larger due to deformation resulting from residual stress. The cantilevered beams 120 are preferably electrically coupled through one or more electrode layers 142, but may alternately be electrically coupled by conductive traces 146, be electrically isolated from one another, or be a blend, wherein some cantilevered beams 120 are electrically coupled while others are electrically isolated. The cantilevered beams 120 may be coupled in series or in parallel, but are preferably coupled with a blend of the two extremes, wherein some cantilevered beams 120 are coupled in series and others in parallel.

Method of Manufacturing an Acoustic Transducer

As shown in FIGS. 5 and 6, a preferred method of manufacturing a transducer can include: depositing alternating layers of piezoelectric and electrode onto the substrate in block S100, processing the deposited layers to define cantilever geometry in block S200, depositing metal traces in block S300, and releasing the cantilevered beams 120 from the substrate 100 in block S400. As the transducer 100 is preferably fabricated using standard CMOS processes, associated electronics (e.g. a JFET, charge amplifier, integrated circuit) may be fabricated on the same substrate as the transducer 100 using the same CMOS process.

Block S100 of the preferred method recites depositing alternating piezoelectric and electrode layers onto the substrate. Block S100 preferably functions to create the layers of the cantilever. The piezoelectric layers preferably include aluminum nitride (MN) due to its CMOS compatibility, but may alternatively include lead zirconate titanate (PZT), zinc oxide (ZnO), polyvinylidene fluoride (PVDF), lead mangnesium niobate-lead titanate (PMN-PT), or any other suitable piezoelectric material. The electrode layers preferably include molybdenum (Mo), titanium (Ti), aluminum (Al) or platinum (Pt), but may alternately include any other suitable electrode material. The cantilever is preferably manufactured using surface micromachining, but may alternatively be manufactured by bulk micromachining. Each layer is preferably deposited onto the previous layer (wherein the first layer is deposited onto a SiO₂ layer), then etched into a desired pattern before the next layer is deposited. Each layer is preferably deposited by thin film deposition, but may alternately be deposited by reactive physical vapor deposition, physical vapor deposition, chemical vapor deposition, epitaxy, or any suitable process. Each layer is preferably first patterned by photolithography, then micromachined to remove the material in the areas exposed by photolithography. Micromachining methods may include wet etching (chemical etching) and dry etching (e.g. through reactive ion etching or ion milling), but may include any other suitable etching method. In one embodiment, the electrode layers are patterned such that alternating layers are staggered (as shown in FIG. 6), such that every other electrode layer can be coupled in parallel by the metal traces deposited in S300. However, the electrode and piezoelectric layers can be patterned with any suitable pattern.

Block S200 of the preferred method recites processing the deposited layers to define cantilever geometry. Block S200 preferably functions to create gaps that define the gap-controlling geometry of the cantilever. The deposited layers are preferably processed by etching gaps through the deposited layers (e.g. with reactive ion etching, wet etching, ion milling, or any other etching method), but may alternately be otherwise processed to define the cantilevered beams 120 and release them from their neighbors. The gap thicknesses are preferably 1 micron or less, but may alternately be slightly larger. Additionally, the gaps preferably bisect each other to form substantially triangular cantilevered beams, but may alternately intersect at the ends to form the desired gap-controlling geometry. This step preferably creates at least two bisecting gaps, such that at least four triangular cantilevered beams are formed, but may alternately create three, four, or any number of gaps to form any number of cantilevered beams.

Block S300 of the preferred method recites depositing metal traces. Block S300 preferably functions to electrically couple the acoustic transducer to one or more amplifiers. Block S300 can occur before, after, or concurrently with block S200. The metal traces are preferably deposited as a layer then patterned, but may alternately be pre-patterned and deposited onto the acoustic transducer. Block S300 preferably provides a metal trace for each electrode or electrode layer 142, but may provide a single metal trace for a plurality of electrodes, wherein the electrodes are coupled together in parallel. The metal traces preferably extend through the intermediary piezoelectric and/or electrode layers to the relevant electrode layer 142, but may be alternatively coupled to the transducer electrodes in any suitable manner.

Block S400 of the preferred method recites releasing the cantilevered beams from the substrate. Block S400 preferably functions to allow the cantilevered beams to expand, contract or bend as necessary to substantially relieve residual stress. The cantilevered beams are preferably released from the substrate by removing the substrate from underneath the cantilevered beams. This is preferably accomplished using DRIE (deep reactive ion etching), but may be accomplished using wet etching, EDM (electric discharge machining), micromachining processes, or any other processing method that releases the cantilevered beams from the substrate. Alternatively, the cantilevered beams can be entirely released from the substrate and subsequently reattached, either to the same substrate or to a different substrate. The cantilevered beams can be entirely released by providing a sacrificial layer between the substrate and the cantilevered beam layers before beam layer deposition (i.e. before block S100), and subsequently etching away the sacrificial layer in block S500. The sacrificial layer is preferably an oxide, but may be any suitable material dissimilar from the piezoelectric and electrode layer materials that may be selectively removed. The sacrificial layer is preferably etched away with an etchant, such as hydrogen fluoride (HF) in an aqueous solution, plasma etching, or any other suitable etching process. The cantilevered beams are preferably reattached to a substrate along their bases by electrostatic clamping or any suitable technique.

The preferred method can additionally include the step of growing an oxide layer on the substrate in block S500. Block 500 preferably occurs prior to block S100, and preferably functions to control the amount of cantilever beam release in S400. In one variation of block S400, the substrate removal process preferably ends at the oxide layer. In a second variation of block S500, the oxide layer preferably functions as the sacrificial layer. A suitable oxide is preferably grown over the desired active area of the transducer, but may alternatively be grown in the desired release areas of the transducer, over the entire substrate or in any suitable area. The oxide is preferably an oxide grown from the substrate, more preferably silicon dioxide (SiO₂), but may be any suitable oxide that is grown or deposited on the substrate. The oxides are preferably grown using general thermal oxidization, but may alternatively be grown using plasma-enhanced chemical vapor deposition (PECVD oxide deposition), chemical vapor deposition (CVD oxide deposition), physical vapor deposition (PVD oxide deposition), or any other suitable oxidization or oxide deposition process. The preferred method can additionally include removing the oxide layer in block S500A, which removes the oxide layer from the transducer by etching or micromachining.

The preferred method can additionally include depositing a seed layer in block S600. The seed layer preferably functions as an active layer on which to build the cantilevered beams. Block S600 preferably occurs before Nock S100. More preferably, block S600 is performed after block S500 such that the seed layer is arranged between the piezoelectric or electrode layer of the cantilevered beam and oxide layer. The seed layer is preferably aluminum nitride (AlN), but may be any suitable piezoelectric, electrode, or seed material. The seed layer is preferably sputtered using physical vapor deposition (PVD) or any other suitable sputtering technique, but may be otherwise deposited over the oxide layer or substrate.

Example Method and Transducer

As shown in FIGS. 6A-6H, one example implementation of the preferred method includes growing a thermal oxide (SiO₂) on the substrate in block S500, depositing an aluminum nitride (AlN) seed layer in block S600 (FIG. 6A), depositing and patterning a first electrode layer (molybdenum) (FIG. 6B), depositing and patterning a first piezoelectric layer (MN) (FIG. 6C), depositing and patterning a second electrode layer (mobidum) (FIG. 6C), depositing and patterning a second piezoelectric layer (AlN) (FIG. 6D), and depositing and patterning a top electrode layer (molybdenum) (FIG. 6D) in block S100. Cavities can preferably be etched through the piezoelectric layers (AlN vias) to the electrode layers, and the metal traces deposited in block S300 (FIGS. 6E and 6F). In one variation, two metal traces are deposited, wherein the first cavity/metal trace extends to and couples with the top and bottom electrode, and the second cavity/metal trace extends to and couples with the middle electrode. The cantilever beams are defined from the deposited layers S200 (etched or micromachined) (FIG. 6E), and freed from the substrate by etching the substrate from the backside with deep reactive-ion etching (DRIE) in block S400 (FIG. 6G). The DRIE stops on the oxide layer, and the oxide layer is removed to release the transducer from the substrate S500A (FIG. 6H).

Throughout performance of the example method, the residual stress is preferably monitored using wafer curvature measurements (e.g. through optical or physical measurements), but may alternately be measured by stress measurements (e.g. stress transducers), nonlinear elastic stress measurements (e.g. ultrasonic or magnetic techniques, X-ray or neutron diffraction), or any other method of measuring the residual stress or curvature in the cantilever. The deposition parameters are then preferably adjusted to minimize the cantilever deflection or stress.

As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims. 

1. A MEMS transducer comprising: a substrate; a plurality of adjacent, tapered cantilevered beams, each defining a beam base, a beam tip, and a beam body disposed between the beam base and the beam tip, arranged such that each of the beam tips extends toward a common area, and further such that each beam is joined to the substrate along the beam base and is free from the substrate along the beam body.
 2. The transducer of claim 1, wherein a gap defined along the length of adjacent beams is approximately 1 micron or less.
 3. The transducer of claim 1, wherein the common area is an imaginary point, wherein the beam tips substantially converge at the imaginary point.
 4. The transducer of claim 1, wherein the beam bases form a substantially continuous microphone perimeter.
 5. The transducer of claim 4, wherein each beam comprises a triangle, such that the microphone comprises a substantially polygonal perimeter.
 6. The transducer of claim 4, wherein the transducer comprises two opposing, trapezoidal beams and two opposing, triangular beams, wherein the short parallel sides of the trapezoidal beams and the tips of the triangular beams converge in a common central area, wherein each angled side of each trapezoidal beam is adjacent a side of a triangular beam.
 7. The transducer of claim 1, wherein at least two of the beams are electrically coupled together in series.
 8. The transducer of claim 1, wherein each beam comprises alternating piezoelectric and electrode layers.
 9. The transducer of claim 8, wherein the beams include two piezoelectric layers and three electrode layers.
 10. The transducer of claim 9, wherein the electrode layers of each beam are electrically coupled in series by metal traces.
 11. transducer of claim 8, wherein the piezoelectric layers comprise aluminum nitride, and the electrode layers comprise molybdenum.
 12. An acoustic sensor comprising: a substrate; a plurality of tapered cantilevered beams, each beam comprising a base fixed to the substrate and each beam tapering from the base into a movable tip, the plurality of beams arranged adjacently with each movable tip converging in a common area such that the bases form a substantially continuous microphone perimeter; and each beam further comprising alternating piezoelectric and electrode layers.
 13. A method comprising: depositing a piezoelectric layer; patterning the piezoelectric layer; depositing an electrode layer; pattern the electrode layer; processing the deposited layers to define a plurality of tapered beams each defining a beam tip extending toward a common area; depositing metal traces; patterning the metal traces; and releasing the beams from a substrate.
 14. The method of claim 13, wherein adjacent tapered beams define gaps extending through the thickness of the deposited layers.
 15. The method of claim 14, wherein a gap width between adjacent tapered beams is approximately 1 micron or less.
 16. The method of claim 14, wherein the gaps are micromachined.
 17. The method of claim 14, wherein the beams are defined by two intersecting, linear gaps.
 18. The method of claim 13, wherein depositing the metal traces comprises depositing a metal trace to each electrode layer.
 19. The method of claim 13, wherein releasing the beams from the substrate comprises etching the substrate away from the beams.
 20. The method of claim 19, wherein etching the substrate comprises deep reactive-ion etching.
 21. The method of claim 20, further comprising growing a thermal oxide layer on the substrate before depositing and patterning the piezoelectric and electrode layers. 